Mikel Oyarzabal slotted home a late penalty to fire Real Sociedad into the Copa del Rey final with a 1-0 win over Basque rivals Athletic Bilbao on Wednesday, securing a 2-0 aggregate semifinal triumph.
American coach Pellegrino Matarazzo has turned La Real's fortunes around since arriving in December and his side will face Atletico Madrid in the Seville final on April 18, after they ousted Barcelona.
Already holding a 1-0 lead from the first leg at Athletic's San Mames, Real Sociedad produced a sturdy display at the Reale Arena to knock out the 24-time winners.
They have now not lost in their last 10 derby clashes at home against Athletic, whom they beat in the 2020 final, and rarely looked like letting their advantage slip.
La Real had the better of a tense first half, with Carlos Soler coming closest. The midfielder's free-kick, flying towards the top corner, was tipped over by Athletic goalkeeper Alex Padilla.
Matarazzo's team had more of the ball and forced the visitors back, albeit without carving out many more openings.
Athletic defender Aitor Paredes made a last-ditch block to keep former Valencia midfielder Soler at bay, and Goncalo Guedes drilled into the side-netting.
Ernesto Valverde's side improved in the second half and began to threaten La Real, again without finding a clear sight of goal.
Alejandro Berenguer fizzed a shot wide after Inaki Williams fed him on the edge of the box.
The match was decided from the penalty spot when Athletic's Inigo Ruiz de Galarreta grabbed a fistful of Yangel Herrera's shirt as he tried to jump in the box.
After a VAR review the referee awarded a spot-kick and Spain striker Oyarzabal coolly sent Padilla the wrong way in the 87th minute.
Mikel Vesga might have levelled on the night for Athletic in stoppage time but Real Sociedad stopper Unai Marrero saved well with his leg to help book his team's flight to Andalusia.
Icelandic striker Orri Oskarsson might have extended La Real's lead at the death but nodded against the post, although it did not matter in the end.